In pixel units of various display devices, a thin film transistor (TFT), in which a display device is driven by applying a driving voltage, is widely used. The active layer of the TFT are normally made of amorphous silicon (a-Si) material having better stability and processability, but the a-Si material has a small carrier mobility, which cannot satisfy requirements of display devices with a large size and a high resolution, especially cannot satisfy requirements of next generation of active matrix organic light emitting device (AMOLED). Compared to the a-Si thin film transistor, a polycrystalline silicon thin film transistor, especially a low temperature polycrystalline silicon thin film transistor has a larger carrier mobility, a better liquid crystal property and a smaller leakage current, so it has gradually replaced the amorphous silicon thin film transistor, and become the mainly used thin film transistor.
The manufacture process of the polycrystalline silicon thin film transistor may be classified as two types: one type is a high temperature process, wherein the temperature is larger than 600° C. in the manufacture procedures, and the substrate is made of an expensive quartz; the other type is a low temperature process, the temperature is smaller than 600° C. in whole manufacture procedures, and the substrate is made of an inexpensive glass. Therefore, the low temperature polycrystalline silicon (LTPS) technology has gradually replaced the amorphous silicon technology of developing the thin film transistor. In the procedure of manufacturing the low temperature polycrystalline silicon, a challenge of crystallization of polycrystalline silicon is existed as usual.